One problem which occurs in a great number of coal gasification systems, in particular atmospheric entrained flow gasification systems, is the difficultly experienced when attempting to recycle char back into the gasification vessel for further reaction. Char, as defined herein, is a particulate compound consisting of carbon and inert ash which results from incomplete gasification of the fresh feed coal in the coal gasifier vessel. This char, consisting of approximately 50% carbon and 50% ash, must be recycled back into the gasification vessel for further reaction to maintain system efficiency at an acceptable level.
In a two stage entrained flow gasifier, the first or slagging stage fires fresh coal and char with an oxidant stream, such as air, to not only provide the heat required to drive the gasification reaction in the second stage, but also to remove the inert ash compounds present in the coal as a liquid slag which may be drained from the first stage by gravity and solidified in a cooling tank located below the first stage. This first stage is usually operated under substoichiometric conditions with temperatures in the range of (1371 to 1927 C.) in order to maintain the slag in a liquid state.
The above mentioned problem arises due to the relatively low reactivity of the char being fed to the first stage when compared to the reactivity of the frsh coal feed. Fresh coal includes only a small portion of inert ash (generally no more than 10% by weight) and a volatile fraction of reactable hydrocarbons. Char, on the other hand, typically has five times the relative amount of inert material and no reactable hydrocarbons or volatile matter. It is thus common for the recycled char to not react effectively in the first stage of the coal gasifier vessel causing a large amount of unreacted material to enter the second or gasification stage. This large char recycle rate results in overloading of the down stream particulate removal and recycle feed equipment and can severely penalize system efficiency. Moreover, insufficiently heated char ash not leaving the first stage can become viscous and cause plugging of the slag tap, necessitating shutdown of the gasifier.
What is needed is a simple, effective means for inducing the char to react with oxygen present to form carbon monoxide product gas and to cause liquefaction of the ash component of the char in order to facilitate the removal of slag from the first stage.